1. Field of the Invention
This invention relates generally to the cleaning of steam generators and, more particularly, to an improved method and apparatus for removing sludge from the tube sheet of a nuclear steam generator.
2. Description of the Prior Art
Nuclear power plants typically utilize steam generators having a vertical inverted U-shaped tube bundle which carries the primary water directly heated by the nuclear reaction. Feedwater is carried by the shell side of the exchanger in contact with the tube bundle for generating steam to be directed to steam turbines.
Among the maintenance problems that can arise in such nuclear power plants, some of the most potentially troublesome include sludge build-up in the steam generator, and particularly relate to concentrations of sludge which may accumulate on the tube sheet at the lower end of the tube bundle.
This accumulation of sludge lowers steam production capacity, and the particles in the feedwater can cause abrasion of the U-tubes in the upper portions of the steam generator. These solids may even cause the steam turbine to foul if they are carried over in the steam. Also, since water chemistry cannot be controlled within the sludge piles, the steam generator tubes may corrode or dent.
Several problems are caused by damaged tubes. Primary water from the tube bundle may leak into the feedwater that is to be turned into steam, thus creating a safety hazard. Plugged and sleeved tubes reduce the heat transfer area of the steam generator. As more time is required to be allotted to maintenance, more radiation exposure is required for maintenance personnel. Also the steam generator's productive life span can be decreased significantly.
There are several U.S. patents disclosing various methods and apparatuses for the sludge lancing of steam generators:
U.S. Pat. Nos. 4,079,701 and 4,276,856 disclose a high pressure, low flow single movable lance system and method that require a fluid flushing stream continuously maintained from the pair of stationary flushing fluid injection nozzles inserted in one hand hole of the steam generator, around the annular space between the lower shell of the steam generator and the tube bundle, to a flushing fluid suction apparatus located diametrically opposite the first hand hole at a second hand hole. While the fluid flushing stream is continuously maintained, the single movable fluid lance is placed in the steam generator through the first hand hole and moved along the tube lane to dislodge sludge deposits from between the tube rows and move the sludge outward into the annular space where the sludge is entrained in the continuously flowing flushing fluid stream.
U.S. Pat. Nos. 4,445,465 and 4,498,427 disclose improved sludge lancing systems which alternately direct the entire fluid flow first to the single movable lance for dislodging the sludge from between the tube rows and moving it outward to the periphery of the tube bundle, and then to a stationary flushing fluid injector which directs the entirety of the available fluid around the periphery of the tube bundle to flush the sludge which was dislodged in the previous lancing cycle.
U.S. Pat. No. 4,424,769 discloses a sludge lancing system in which two streams of cleaning fluid under a high pressure are directed from the end portion of a lance toward the tubular plate between two parallel sheets of tubes and in directions which are fixed and symmetrical with respect to the direction of the two sheets of tubes. A mechanism is provided for cutting off the stream of high pressure water as the water jets confront the tubes during radial movement of the lance.
U.S. Pat. No. 4,452,183 discloses a sludge lancing system in which two high pressure water jets are operated simultaneously from opposite hand holes to insure that the sludge is moved to the periphery without redeposition of the sludge in areas of the steam generator. Furthermore, there is required simultaneous evacuation throughout the length of the zone in order to remove the sludge.
U.S. Pat. No. 4,527,515 discloses a control system for a single movable lance sludge removal system, wherein a valve is provided for directing a pressurized fluid to either a jetting outlet or a flushing outlet.
U.S. Pat. Nos. 4,273,076 and 4,492,186 disclose additional single movable lance sludge removal systems.
In addition, British Pat. No. 315,446 discloses a method and apparatus for removing sand from a casting, wherein two nozzles are mounted one above the other, with one nozzle emitting a very high pressure cutting stream, and the other nozzle emitting a lower pressure rinsing stream.
FIG. 1 illustrates a known sludge lancing process and apparatus and the problems associated therewith:
A. Access to tubesheet 10 was through 6"-handholes 12 and 14 (each end of blowdown lane 16). PA1 B. High pressure lance 18 and suction system 19 were both inserted into the same 6"-handhole 12. PA1 C. Two flexible, perforated suction headers, 20a and 20b were placed along the periphery 22 of the tube bundle 24. PA1 D. Two high velocity jets 26 and 28 of water were established from the lance 18. PA1 E. Sludge redeposition or wash back into the bundle occurred due to the high volume of water which exited the bundle and which overwhelmed suction header capacity. PA1 F. During lancing of second side of generator via handhole 14, washback of sludge occurred in the first side lanced. PA1 G. Flexible headers 20a and 20b required a large number of holes, thereby precluding sufficient suction near the outer ends of the headers. PA1 A. An injection header 30 and a suction header 32 are respectively inserted diametrically opposite handholes 14 and 16 near the elevation of the tubesheet 10, thereby causing a circumferential fluid stream 34 to be established from the injection header 30 around the tube bundle to the suction header 32. PA1 B. A fluid lance 36 is moved along the line between the injection and suction headers, while emitting a fluid jet 38 perpendicular to the movement of the fluid lance. PA1 C. The fluid jet forces sludge to periphery where it is entrained in and carried away by the circumferential stream 34. PA1 D. Thus, this patent discloses a sludge removal method that uses a fluid lance in conjunction with a peripheral fluid stream.
FIG. 2 illustrates the essence of the U.S. Pat. No. 4,079,701 and the process and apparatus disclosed therein in an attempt to overcome the above problems: